<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<script>
			const arr = [
				[1, 2],
				[3, 4]
			]
			const ExtendArr = arr.map((val) => {
				const [a, b] = val
				console.log(val, a, b, "sum")
				const sum = a + b
				return sum
			})
			console.log(ExtendArr, "ExtendArr")

			const cArr = [1, 3, 5]
			cArr[7] = 7
			// [1, 3, 5, undefined, undefined, undefined, undefined, 7]
			console.log(cArr, "cArr")
			console.log(cArr[3], "cArr[3]")
		</script>
	</body>
</html>